All bodies above absolute zero radiate. When this radiation is able to come into thermal equilibrium with the bodies emitting it, the radiation has a characteristic spectrum, the Planck spectrum, which depends only on the temperature T of the system. Such radiation is called blackbody. It has an energy density aT4 [J m-3]. The energy per unit time per unit area emitted by a body in thermal equilibrium with the radiation is σT4 [W m-2]. The average energy of a blackbody photon is 2.7 kT.